$notification = array
(
'body'   => 'test',
'title'     => 'test',
"icon" =>  'notification_icon',
"content_available" => "1",
"force-start" => 1
);

$data = array
(
"title" => "test",
"body" => "test",
"icon" => "notification_icon",
"content_available" => "1",
"force-start" => 1
);

$fields = array
(
'registration_ids'  => $registrationIds,
'notification'      => $notification,
'data'          => $data,
"priority" => "high",
"content_available" => true
);